The invention relates to phaselock loops and phase comparators and, more particularly, to high precision phaselock tracking loops and phase comparison systems for fixed-frequency signals which may be deeply embedded in noise.
Precise phaselock tracking of noise-contaminated fixed-frequency signals has important applications in radionavigation, in the reception of time reference signals, and in other fields. With respect to time reference signals, the National Bureau of Standards and similar organizations in foreign countries broadcast signals which are phase-locked to standard time. When received at locations remote from the transmitters, these signals may have noise levels so high that the accuracy of tracking such signals by prior art phaselock loop apparatus is severely limited. In radionavigation systems such as Decca, Loran C and Omega, radio signals received from several ground transmitters are tracked and phase-compared to calculate the position of the receiver. Because the operating ranges of such transmitters may be very long (of the order of 6,000 miles in the Omega Navigation System, for example), the accuracy of the calculated position information depends upon the ability to precisely track weak signals which may be deeply embedded in noise.
In certain applications accurate phase tracking of noise-contaminated signals is further complicated by the discontinuous or intermittent nature of the signals. For example, the radio signals in the Omega Radionavigation System consist of pure continuous wave emissions for periods of approximately one second followed by nine seconds of silence.